Various methods are known for limiting diesel engine exhaust emissions of oxides of nitrogen (NOx), particulate matter, and hydrocarbons using diesel oxidation catalysts (DOC), particulate filters (DPF), oxides of nitrogen absorbers (LNT), and/or selective catalyst reduction (SCR) components positioned downstream of a turbocharger. However, exhaust temperatures may be lower and difficult to control when such exhaust aftertreatment devices are positioned downstream of the turbocharger. For example, such a positioning may require additional fuel to be oxidized in the DOC to activate the LNT and/or SCR and to regenerate the DPF. Further, LNT and SCR oxides of nitrogen (NOx) conversion efficiencies may be highly dependent on temperature.
The inventors herein have recognized that by placing an emission control device in between two exhaust turbines, desired exhaust temperatures may be more easily achieved. Further, exhaust temperature may be more directly controlled, such as by the intake air throttle, fuel injection timing, and exhaust pressure. The exhaust pressure may be controlled when the turbines are variable geometry turbines (VGT), include wastegate valves, variable nozzles, etc. By adjusting the VGT, wastegate valves, etc. the amount of expansion of the exhaust gas via the turbines may be controlled.
In one example, a method is provided for controlling an engine exhaust system having a first and second turbine, and with a temperature-dependent emission control device coupled between the first and second turbine. The method comprises: during a warm-up condition, operating with a decreased expansion across a first turbine positioned upstream of the emission control device and an increased expansion across a second turbine positioned downstream of the emission control device; and during at least one condition after the warm-up, operating with an increased expansion across the first turbine and a decreased expansion across the second turbine. Further, during the warm-up condition, the first turbine may be adjusted to increase expansion across the first turbine in response to a request for increased engine output, e.g., in response to a driver tip-in, when the second turbine is at a maximum level of expansion.
In such an approach the temperature of the exhaust, DPF, LNT, and/or SCR during non-warmed exhaust conditions, e.g. after engine cold starts from rest, may be substantially raised. Such a temperature increase in the emission control device may also increase NOx conversion efficiencies typically lost during a system warm up. Further, the exhaust temperature control of such an approach may eliminate the need to oxidize fuel in the DOC to control the temperature of the exhaust, DPF, LNT, and/or SCR, thus eliminating either fuel in oil dilution caused by late post fuel injection or the use of a separate fuel injector in the exhaust. As such, fuel economy may be improved. Additionally, the component size and precious metal loadings of the aftertreatment devices may be reduced due to higher temperatures in the aftertreatment devices at comparable engine speeds, loads, and warm up times.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.